The invention relates to a process and to an installation for producing and recording ultrasonic sectional images, whereby an examination object is scanned line-by-line with ultrasonic transmission signals and the developing echo signals are correspondingly recorded on the recording medium of an image recording installation to form a visible image. By recording medium of an image recording installation, the screen of an oscillograph tube is particularly to be understood. However, in the broadest sense, the recording media of recorders of any type are also included which, on the basis of line-, point-, droplet recording or etc., produce visible images on the medium, which are modulated in their optical density structure or color structure, too, as a function of the intensity of developing echo signals.
In known sectional image recording devices, the effect arises that in medium image areas and image areas near the skin, the dynamic range of the electric echo signals is significantly higher than that amplitude range which can be reproduced with intensity modulation of the recording beam in the visible image of the recording installation. Thereby, however, significant information is lost which could contribute to a better image interpretation. If, for instance, the image recording ensues with low amplification, then specific structures in the median image area and in the image area near the skin can be depicted well in outline but already less well in the inner fine structures. One obtains the latter information when the amplification is increased. Thereby, however, image areas that are already well structured are swamped and thus lose their richness of detail or their resolution. In view of this disadvantage, an image reproduction is desirable which unites in a single image those diagnostically valuable image parts of the recording image obtainable with low amplification and those obtainable with higher amplification.
Such a unification to a total image could indeed be attained with a photographic trick by means of so-called dodging (and/or burning-in) technology, where, upon producing a positive image in the photolab, those places of an ideal negative in which an overexposure is to be feared are covered (are dodged) for a certain time. Thus, details are retained which would otherwise be submerged in the black image passages. This technique, however, is rather expensive in comparison to the degree of image improvement attained. Moreover, in a visible image obtained with low amplification it must be expected that the contour will break up because the echoes of structures which are not struck perpendicularly decrease, as a function of the angle, very quickly the more tangential the sonic beam strikes. The practitioner therefore generally uses a different process, which is based on the fact that he changes the amplification during the examination at varying applicator positions and photographs the visible image which is respectively produced. The subsequent comparison of the various photos then allows the diagnostic statement. As long as attention is being directed toward a limited image area, this procedure is certainly acceptable since it is not connected with costs that are all too high. If, however, an ultrasonic sectional image is to be evaluated in its totality, then disadvantages that are not insignificant result. The human brain as a memory and computer is now placed under significantly higher demands. The demands can indeed be reduced in that the image adjustment is always repeatedly manually changed (wobbled) so that at least the process of remembering is assisted. This procedure, however, is only a crutch which takes time and also, particularly, renders the documentation more difficult.
Further possibilities for improving the visible image ensue in that one either works with color coding in the visible image or with dynamic compression in the echo reception signal. Color coding, however, easily leads to misinterpretations, since each color change in the visible image is automatically interpreted by the human eye in a contour. The dynamic compression indicated could be undertaken by means of non-linear amplitude distortion, for instance logarithmizing at a diode. The problem, however, results that then all interference effects, particularly also the unsharp temporal limitations of the scanning beam including the side lobes are greatly boosted, so that specifically the cross resolution deteriorates.
From the periodical "Fernseh- und Kino-Technik", No. 11/1976, Pages 388 to 392 or, also from "nachrichten elektronik" 1, 1977, Pages 11 and 12, a local-adaptive image processing process for brightness equalization of unevenly illuminated television images is known in the area of television technology. Specifically in this case, the dynamic problem ensues for example in the case of images with strong shadows. Upon an optimum adjustment of the bright part of the picture, details can no longer be seen in the shadow area. That is alleviated by means of dynamic compression which renders a brightening of the shadow area possible. To that end, the integral brightness of the image point to be represented is measured in a surround field by means of a second television picture that is taken unsharp and the brightness of the point is boosted in the inverse ratio to the measuring result. In the case of a very dark surround field, this thus means a strong boost; in the case of a bright surround field a corresponding weaker boost. In this manner, thus, a reduction of the image dynamic range ensues without influencing the detail contrast of the image.
Such a solution applied to the ultrasonic sectional image would, thus, for example, assist the orientation in the search process (for example, pancreas, placenta), because the adjustment of the image parameters (amplification, depth compensation) is less critical. Localization diagnostics would be significantly enriched because of the improved representation of curved organ boundaries. The resolution (image sharpness) would remain unaffected in this type of compression because the amplitude relationship of the desired to the undesired image points (of side lobes) remains unaltered. Thereby, the surround field would have to be selected large enough so that echoes of main and side lobes can be simultaneously comprehended. Then the main lobe echo would ensure that the intensification of the side lobe echoes remains small and that these are not accentuated. Only such structure echoes which lie at a greater lateral interval would be prepared out of the relatively dark surround field by means of amplification boosting. One could then think of transforming the signals of the ultrasonic image in television standard (BAS) and using a television set for representation which functions according to the process of dynamic compression described above. That, however, presupposes that the rectification and data conversion must be executed in a significantly greater dynamic range than is necessary for a simple, uncompromised representation. However, specifically in view of the speed of the processes, this causes problems, so that the transduction would become expensive.